1. Field of the Invention
The present invention is directed generally to time of arrival estimation and, more particularly, to a system and method for mathematical modeling of a function indicative of time of arrival.
2. Description of the Related Art
Existing position location technologies based on global positioning system (GPS) use a network of satellites in the sky which transmit signals at a known time. A GPS receiver on the ground measures the time of arrival of the signals from each satellite it can detect. The time of arrival, along with the exact location of the satellites and the exact time the signal was transmitted from each satellite is used to triangulate the position of the GPS receiver. A typical GPS receiver requires four satellites to make a triangulation, and the performance of the resulting calculation increases as the number of satellites that can be detected increases.
In an alternative to GPS, an existing network of cellular base stations can be treated as a network of satellites for purposes of position location. Similar to GPS technology, the exact location of each base station, the exact time at which the base station is transmitting a signal, and the time of arrival of the base station signal at a mobile station can be used to triangulate the position of the mobile station. This technique is described by some service providers as advanced forward link trilateration (AFLT). Wireless networks may also be used in conjunction with GPS to determine the location of the mobile station.
A significant problem faced by the mobile station is to measure the time of arrival of the signals that are received from each base station. Different wireless technologies may take different approaches to time of arrival measurements. Code division multiple access (CDMA) is one such technology. CDMA modulation is one of several techniques that allow a large number of system users to share a communication system. It is possible to utilize conventional CDMA modulation techniques as part of an AFLT system.
CDMA modulation techniques are disclosed in U.S. Pat. No. 4,901,307, issued on Feb. 13, 1990, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” which is assigned to the assignee of the present invention, and the disclosure of which is incorporated herein by reference. The above-referenced patent discloses the use of a phase-coherent and chip-synchronous chip sequence that is defined as a pilot chip sequence, or pilot signal. The pilot signal can be used to provide phase and time acquisition and tracking, and multi-path correction.
Methods for acquiring the pilot signals are disclosed in the above-referenced patent and in the following patents: (1) U.S. Pat. No. 5,781,543, issued on Jul. 14, 1998 and entitled “POWER-EFFICIENT ACQUISITION OF A CDMA PILOT SIGNAL;” and (2) U.S. Pat. No. 5,805,648, issued on Sep. 8, 1998 and entitled “METHOD AND APPARATUS FOR PERFORMING SEARCH ACQUISITION IN A CDMA COMMUNICATION SYSTEM,” both of which are assigned to the assignee of the present invention and the disclosures thereof are incorporated herein by reference.
When the wireless communication device is first powered on, the device must establish a communication link with a base transceiver station (BTS). The wireless communication device will typically receive pilot signals from a plurality of BTSs. The wireless device will search for the signals from the BTSs and will establish a communication link with a selected BTS to permit the reception and transmission of data, such as audio signals, over the established communication link. The selection of a particular BTS and the actual communication between the wireless communication device and the selected BTS are well known in the art and need not be discussed in detail herein.
As discussed in the above-referenced patents, each BTS broadcasts the same pseudo-noise (PN) code pilot signal, but with a different time offset.
To acquire the pilot signal, the wireless device must synchronize with the time offset and frequency of the signal transmitted by a BTS. The object of a “searcher” process in the wireless device is to find the time offset of the received signal. The searcher uses an estimated frequency. If the estimated frequency is not sufficiently close to the frequency of the pilot signal, the received signal will not be acquired.
When a BTS is properly detected, the output of the searcher is a pulse, which may be considered a correlation pulse. This correlation pulse may be used to measure the time of arrival of the signal from the BTS. However, the accuracy of such time of arrival measurements severely limits the accuracy of the position determination. Therefore, it can be appreciated that there is a significant need for an improved system for estimating time of arrival that increases the accuracy of the position determining process. The present invention provides this and other advantages, as will be apparent from the following detailed description and accompanying figures.